THE EYES. 67 



the nerve in the Araneid eye always joins the optic cells at the 

 ends at which the nuclei lie (Fig. 34 A and B). In the present 

 «ase, this is the distal side of the retina ; the nerve has therefore 

 retained its original position. 



It appears, from Bertkau's account, that the nerves join the distal (external) 

 ends of the optic cells (Fig. 34 B), the nerve-fibres running from the periphery 

 to the ends of the cells. This seems to be the most primitive arrangement. 

 Beetkau, however, states that nerve -fibres also traverse the tapetum (Fig. 

 34 B) and run to the distal ends of the cells {i.e., in between the optic cells). 

 This latter course taken by the nerves must be regarded as secondary, if we 

 consider the processes of development, for a perforation of the post-retinal layer 

 then takes place. 



It is possible to reconcile Bertkau's description with the views founded by 

 Mark upon ontogenetic researches. According to these, some of the nerve- 

 fibres run round the tapetum to join the distal ends of the retinal cells ; others, 

 however, are said to traverse the post-retinal layer as well as the tapetum. 



Additional remarks on the development of the principal and the 

 accessory eyes. The statements of Kishinouye, as we have already 

 remarked, would, if confirmed, he of importance in affecting our 

 view of the development of the Araneid eye. According to this 

 writer, the principal eyes arise through inversion, essentially in the 

 manner described above. An important point is found in the fact 

 that the aperture of invagination represents also the last trace of 

 the aperture of the closing cephalic pit. The (anterior) median 

 eyes are thus, as in the Scorpiones, related to the invaginated part 

 of the cephalic lobes, a fact which could not be gathered from the 

 descriptions of earlier authors. 



While the principal eyes arise through inversion, the accessory 

 eyes, according to Kishinouye, have quite another origin, being 

 formed by a mere depression of the ectoderm without inversion. 

 They appear later than the principal eyes, and lie behind these. 

 It appears that they are related to the invaginations mentioned 

 above, which form independently of the cephalic pits, and take part 

 in the formation of the brain. This recalls the formation of the 

 optic ganglia. The accessory eyes seem to arise independently of 

 •one another as pit-like ectodermal depressions. The base of each 

 of the depressions thickens considerably, and becomes the retina. 

 The nerve joins the retina posteriorly, so that there is no suggestion 

 of inversion, and the whole resembles the eye of the Insecta. The 

 pit closes, its lips growing together and fusing. These parts yield 

 the vitreous body, above which the lens is secreted. 



If the Araneid eyes actually form in the manner here described, 



